Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide, and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase. Thus, a process that uses higher alkenes instead of propylene as feed and co-produces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
One such process proceeds via cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts.
Cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. In this process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce a reaction intermediate such as cyclohexene which then alkylates the benzene starting material. Thus, U.S. Pat. Nos. 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts which comprise nickel-treated and rare earth-treated zeolites and a palladium promoter. Similarly, U.S. Pat. Nos. 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed in these prior art processes are zeolites X and Y. In addition, U.S. Pat. No. 5,053,571 proposes the use of ruthenium and nickel supported on zeolite beta as the aromatic hydroalkylation catalyst. However, these earlier proposals for the hydroalkylation of benzene suffer from the problems that the selectivity to cyclohexylbenzene is low, particularly at economically viable benzene conversion rates, and that large quantities of unwanted by-products are produced.
More recently, U.S. Pat. No. 6,037,513 has disclosed that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting the benzene and hydrogen with a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family. The hydrogenation metal is preferably selected from palladium, ruthenium, nickel, cobalt and mixtures thereof, and the contacting step is conducted at a temperature of 50° C. to 350° C., a pressure of 100 kPa to 7000 kPa, a benzene to hydrogen molar ratio of 0.01 to 100 and a weight hourly space velocity (WHSV) of 0.01 hr−1 to 100 hr−1. The '513 patent discloses that the resultant cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide decomposed to the desired phenol and cyclohexanone.
Although the process of the '513 patent represents a significant improvement over earlier processes for the hydroalkylation of benzene, it still suffers from the problem that significant quantities of impurities, particularly cyclohexane and methylcyclopentane, are produced in addition to the desired cyclohexylbenzene. These impurities represent loss of valuable benzene feed. Moreover, unless removed, these impurities will tend to build up in the benzene recycle stream thereby displacing benzene and further increasing the production of undesirable by-products.
One potential solution to this problem is proposed in U.S. Pat. No. 7,579,511, which describes a process for making cyclohexylbenzene in which benzene undergoes hydroalkylation in the presence of a first catalyst to form a first effluent composition containing cyclohexylbenzene, cyclohexane, methylcyclopentane, and unreacted benzene. The first effluent composition is then separated into a cyclohexane/methylcyclopentane-rich composition, a benzene-rich composition, and a cyclohexylbenzene-rich composition and the cyclohexane/methylcyclopentane-rich composition is contacted with a second, low acidity, dehydrogenation catalyst to convert at least a portion of the cyclohexane to benzene and at least a portion of the methylcyclopentane to linear and/or branched paraffins and form a second effluent composition. The benzene-rich composition and the second effluent composition can then be recycled to the hydroalkylation step.
Another solution is proposed in International Patent Publication No. WO2009/131769, in which benzene undergoes hydroalkylation in the presence of a first catalyst to produce a first effluent composition containing cyclohexylbenzene, cyclohexane, and unreacted benzene. The first effluent composition is then divided into a cyclohexylbenzene-rich composition and a C6 product composition comprising cyclohexane and benzene. At least a portion of the C6 product composition is then contacted with a dehydrogenation catalyst under dehydrogenation conditions to convert at least a portion of the cyclohexane to benzene and produce a second effluent composition which comprises benzene and hydrogen and which can be recycled to the hydroalkylation step.
Both of the processes disclosed in U.S. Pat. No. 7,579,511 and WO2009/131769 rely on the use of a dehydrogenation catalyst comprising a Group VIII metal on a porous inorganic support such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, activated carbon, and combinations thereof. However, the conditions that favor methylcyclopentane conversion also favor isomerization of the more prevalent cyclohexane to methylcyclopentane. Moreover, the methylcyclopentane normally converts to acyclic paraffins, such as n-hexane, which forms an azeotrope with benzene making it no easier to separate from the effluent than the methylcyclopentane. Finally, methylcyclopentane has a considerably higher octane value than its acyclic paraffin conversion products, making it more valuable to recover the methylcyclopentane as a motor gasoline blendstock rather than consume valuable hydrogen in its dehydrogenation.
There is therefore a need for an improved process for reducing the build-up of methylcyclopentane in the benzene recycle streams present in benzene hydroalkylation processes.